Q&A - P3 Science

[Lynn2]

Hi atutor2001


Thanks for the details explanation but I do not know how to simplify it and relate it to a p4 kid. Can you break down as in what is the co relation between salt, melting point, freezing point, ice and water?

Honestly, I am abit lost too...thank you.

atutor2001:

Hi Tang, allow me to try and explain :

(1) Under normal circumstances, when ice and water are in contact, the molecules on the surface of the ice escape into the water (melting), and the molecules of water are captured on the surface of the ice (freezing).

(2) At 0°C, the rate of freezing is the same as the rate of melting. As a result, the amount of ice and the amount of water won't change on average. The ice and water are said to be in dynamic equilibrium with each other.

(3) When salt is added, it will disrupt the equilibrium. The salt will dissolve in the water but it cannot \"freeze\" onto the surface of the ice.

(4) Therefore, there are now fewer water molecules on the liquid side because the salt molecule will take up the place of some of the water. As a result, the number of water molecules freezing into ice decreases. However, the rate of melting from the ice surface remains unchanged. Therefore, the rate of melting from the ice will be faster then the rate of freezing back of water into ice.

(5) Before salt was added, the rates of melting and freezing are equal. As such, the heat needed for the ice to melt is provided by the heat released by the water when it freezes back into ice.

(6) Now, with the rate of freezing being lower than the rate of melting, the heat released from \"freezing\" is not enough to support the rate of melting. So, the heat for melting must come from the salt solution and the ice. This cause the temperature of the ice & salt solution to decrease.

(7) With heat being taken from the salt solution, more water molecules will have less energy and more water will freeze back into ice.

(8) The process continues until a new equilibrium is reached, with the temperature of the ice being \"depressed\".

[atutor2001]

Hi Lynn2


Please don’t explain to P4 how the freezing point is "depressed". It is not in the syllabus of P4.

I think P4 only need to know that :

(1) When salt is added to ice, the freezing point of the mixture will become less than 0 degree C

(2) Now that the freezing point of the "mixture of ice with salt" is less than 0 degree, the ice cannot remain as solid because the temperature of the ice is higher than the new freezing point. As a result, the ice will melt.

[Full.010894Cream]

I was also pondering on how to look at this question from a P4 POV.


1) ice should start at 0 deg C. This rules out (4)

2) water in the tt will start to cool down but will not drop below 0 deg C. Only (3)

[Lynn2]

Hi atutor


thanks and noted.

Hey, can check with ks members here, on http://www.misskoh.com/07/primary4-aitong-science.pdf

question 37 a?appreciate someone can explain to me why .thanks

rgds

[atutor2001]

Lynn2:

Hi atutor


thanks and noted.

Hey, can check with ks members here, on http://www.misskoh.com/07/primary4-aitong-science.pdf

question 37 a?appreciate someone can explain to me why .thanks

rgds

The limewater will turn chalky.
The wooden box does not allow light to pass through.
Without light, the plant cannot photosynthesize but continues to respire.
Carbon dioxide is given out during respiration which turns the lime water chalky.

[tianzhu]

Thank you for your help.


Which properties of the brown gas are shown by the above experiment?
A: It can be compressed.
B: It has no definite shape.
It has no definite volume.

A.A only
B.A and B only
C.B and C only
D.A, B and C only

http://farm4.static.flickr.com/3539/3499368658_ae0f175df7_o.jpg\">

[Lynn2]

One question to ask ks member here…


The table below shows the melting points of substances P,Q,Rand s.

Substance Melting point (deg cel)

p 49
q 5
r 23
s 0

At 20 deg, which of the above substances would have changed from a solid state to a liquid state?
p only q only
p and r only q and s only

Can check what is substance p and r are?

what is 20 deg to 49deg and what is 20 deg to 23 deg as in what state is that?thanks

rgds

[Lynn2]

one more


Below shows the number of days of each stage in the life cycle of an insect.
Egg 10 days
Nymph 12 days
adult male 11 days
adult female 8 days.

How many days would the insect take to become an adult after the egg is hatched?

6 days
8 days
10 days
12 days

My girl answer is 10 days which is wrong.thanks

[watmekiasu]

My answer would be 12 days. It takes 10 days for the egg to hatch which produces a nymph. The nymph takes 12 days to grow to adult stage.

[Full.010894Cream]

Lynn2:

One question to ask ks member here....


The table below shows the melting points of substances P,Q,Rand s.

Substance Melting point (deg cel)

p 49
q 5
r 23
s 0

At 20 deg, which of the above substances would have changed from a solid state to a liquid state?
p only q only
p and r only q and s only

Can check what is substance p and r are?

what is 20 deg to 49deg and what is 20 deg to 23 deg as in what state is that?thanks

rgds

try to relate this to something familiar, eg, ice which melting point is 0 deg C.

when temp is anything higher than its melting point, eg, 20 deg C, the ice (s) will start to melt. Similarly, 20 deg is also higher than q's melting point so it'll also melt.

Since substance p & r only melt at higher temp than 20 deg C, it'll remain as solid.

Not important what they are as this is a test for the understanding of melting point.